This invention relates generally to precision sinusoidal wave test generators, and more particularly to a generator adapted to function as an accurate frequency source for checking and calibrating flowmeters.
In measuring flow rate, it is known to use turbine meters for this purpose, the meter including a turbine blade rotor mounted in a flow tube and actuated by the liquid flowing through the tube. A permanent magnet mounted on the rotor induces an alternating-current in a coil external to the tube, the frequency of the generated signal being proportional to the volumetric flow rate.
For measuring flow rate, it is also known to use vortex-shedding meters of the type disclosed in U.S. Pat. No. 3,116,639, in which an obstacle placed in a flow conduit creates fluidic oscillations whose frequency depends on flow rate. These oscillations are sensed to produce corresponding electrical signals. In Swirlmeters, such as the meter described in U.S. Pat. No. 3,279,251, the fluid to be measured is forced through stationary swirl blades to assume a swirl component which is transformed into precessional movement to create a vortex, the vortex being sensed to produce a signal whose frequency is indicative of flow rate.
Flowmeters of the type which produce a signal whose frequency is proportional to flow rate generally includes so-called secondary instruments adapted to convert the output signal from the primary of the meter into a corresponding current with a prescribed range suitable for industrial process control, the customary current range being 4 to 20 mA. The span of the current range is usually set so that a 20 mA output, the upper end of the range, is developed in response to a specific value of flow, say 1000 gallons per minute. This current output represents a particular signal frequency in the output of the meter primary.
In order to check and calibrate the span of the current output in the secondary of the flowmeter, one requires a precision frequency source capable of simulating the output signal of the flowmeter. Thus if signal frequencies corresponding to a 4 to 20 mA span lie in a range of 1 to 2000 Hz, then the test generator must be capable of producing an output having precise frequencies extending through the flowmeter signal range. Moreover, since nearly all flowmeters generate sinusoidal waves with a certain amount of noise superimposed thereon and in various amplitudes, in order to simulate these flowmeter signals, the test generator must be capable of producing a comparable sinusoidal wave having a noise component.
A further practical requirement for a test generator for flowmeters is that it be portable and battery-operated, for the tests are usually carried out in the field rather than in the factory. Hence, high-quality precision generators of the type available in many industrial laboratories, apart from the fact that they are complicated and costly instruments, are unsuitable for field use in that these generators are relatively bulky and operate from commercial power lines. Moreover, even if such high-quality test generators lent themselves to field use, they would still be unsuitable for purposes of checking and calibrating flowmeters in that they provide pure sine waves without a significant noise content. In other words, the normally commendable qualities of these precision generators preclude their use for flowmeter testing where the desideratum is an impure sinusoidal wave.
Also commercially available are relatively low-cost sine wave generators and frequency synthesizers. While such instruments are capable of generating sine waves, their frequency accuracy is poor or marginal and they cannot be trusted to check and calibrate a flowmeter. Also such inexpensive generators produce relatively clean sine waves and do not fully simulate flowmeter signals.